1. Field of the Invention
The present invention relates generally to semiconductor integrated circuits, and more specifically to formation of a programmable contact structure for such circuits.
2. Description of the Prior Art
Field programmable gate arrays are a type of integrated circuit made up of multiple, connected gates and are known in the art. Field programmable gate arrays (FPGA) may be used to provide custom functions. These, and other types of field programmable logic devices, are programmed by storing programming information into the devices in a nonvolatile manner. This stored information defines the operation of the device. One-time programmable logic devices can be programmed using either fuses or anti-fuses. Anti-fuses are well known and implement logic functions by providing an open electrical circuit between nodes until the anti-fuse is programmed. The anti-fuse may be programmed by applying a high voltage across the nodes, which then creates an electrical short circuit by electrically connecting the nodes.
Anti-fuses have been fabricated by first depositing an interconnect metal layer and then a buffer layer which is typically comprised of amorphous silicon. The sandwiched amorphous silicon will result in high contact resistance and therefore act as an insulator between the interconnect metal layer and an upper metal layer until the anti-fuse is programmed. After a programming voltage is applied, the resistance of the anti-fuse decreased so that larger current can flow through the contact. The difference in these resistances is great enough to be interpreted as logical zeros and ones by the circuitry of the device.
During formation of an amorphous silicon contact, an interlevel insulating layer is deposited over the amorphous silicon and is etched to form a contact via. This via exposes the underlying amorphous silicon. As is known in the art, process variations typically cause the insulating layer to have an uneven topography, with some areas being much thicker than others. Etch rates and times must be calculated to etch through the thickest areas of the insulating layer in order to ensure that none of the insulating layer is left in the contact via. However, even using highly selective etches, some of the amorphous silicon will necessarily be etched away in the via. In some cases, so much of the amorphous silicon may be etched away that a short is formed between the interconnect metal layer and the overlying metal layer. This would result in a particular location behaving as a programmed via when such was not intended. Perhaps worse, just enough of the amorphous silicon layer may be removed to make that location behave unreliably, allowing a completed device to pass testing only to fail in actual use.
It would, therefore, be desirable to provide a programmable contact via method and structure in which the amorphous silicon buffer layer is not damaged during fabrication. It would be further desirable for a process for forming such a structure to be simple, reliable, and compatible with standard processing techniques.